This invention relates generally to a digital line synchronizer comprising part of the incoming line equipment serving a synchronized PCM TDM digital switching exchange, and more particularly, relates to a novel synchronizer system useful to impart a predetermined off-set in memory positions between WRITE vectors and READ vectors.
A master PCM digital switching exchange may typically receive digital information pulse trains originating from a plurality of remote located channel bank equipment stations, respectively. The master switching exchange must then assemble a composite digital pulse train from the received terminal pulse trains so that the composite pulse train can be switched through the master switching exchange. It is popular to employ a master-slave syncrhonization method of obtaining synchronous operation of the transmission and reception of digital pulse information over the transmission loop between the master switching exchange and a selected terminal equipment station. Synchronous operation is provided through eliminating the independent derivation of clock timing signals in the transmit section of the distant terminal equipment and instead, slaving the reference timing of the transmit section thereof to the clock timing signals as generated in the master switching exchange and then reconstructed in the receive station of the remote terminal equipment. Such transmission loops are said to be frequency synchronized when the frequencies of the clock oscillators of the remote terminal stations are slaved (derived from) to the clock oscillator frequency within the master exchange. Moreover, as a particular type of frequency synchronization, when the overall propagation or transmission time delay is equal to an integral multiple of the frame period, the transmission loop is said to be frame synchronized. It is generally understood that frame synchronized transmission of PCM data is required in order to minimize the information storage required by input data buffer memory equipment provided at the master exchange.
The occurrence of time delays in propagation of data between the master and slaved stations is fundamental to data transmission and is commonly referred to as loop delay wherein those unchanging or constant delay characteristics, such as propagational distance, number of repeaters, type of channel bank equipment, that are inherent to a particular loop define an absolute loop delay and such well-known variables as cable temperature, pulse or phase jitter, etc., define so-called variable delay characteristics for the transmission loop. It is known to provide input data buffer memory banks within the incoming line equipment of the master station for the purpose of writing incoming PCM data into said memory banks and subsequently reading out the stored PCM data from said memory banks at a time commensurate with the formation of the aforementioned composite pulse train. It is convenient herein to speak in terms of a WRITE command or vector generated to accomplish the input of PCM data to the input buffer memory bank and a READ command or vector generated to accomplish the output of PCM data from the input buffer memory bank. Fluctuations in the overall loop delay will result in the faster or slower arrival of incoming PCM data and the writing of said data must then fluctuate in like turn so as not to destroy or distort received PCM information samples. Ideally, the writing function is first accomplished and thereafter, the reading function is accomplished for a given sample of PCM data and the time delay therebetween is referred to herein as an "off-set." The WRITE/READ vector off-set is also presented herein as a physical off-set within the input buffer memory bank with respect to the word/bit slot memory which is being written and the word/bit slot memory which is being read where the input buffer memory bank is comprised of a plurality of word slots equal to the number of channels within a frame period. When the WRITE/READ vectors vary with respect to each other, the variance is called slippage, and more particularly, the term slip, as applied to digital switching systems, refers to an information error caused by the overlapping of the WRITE/READ vectors. Overlapping, of course, is in reference to the WRITE vector having slipped to the degree that a particular word slot within the memory bank is again written (inputted) with new PCM data before the original PCM data stored therein could be read (outputted).
In synchronous transmission links, the absolute loop delay causes an initial WRITE/READ vector off-set but only the variations in the off-set caused by the so-called propagational variables such as cable temperature and phase jitter need be compensated for in order to prevent the overlapping phenomenon. These propagational variables have dimensions of both magnitude and frequency wherein the major component of the magnitude variation is related to temperature changes and the frequency variation is related to the jitter paramemter. As is associated with a T1-carrier transmission format of 24-channels having 8 digit PCM coding propagated at a bit rate of approximately 1.544 megabits per second with a frame period of some 125 microseconds, the phase delay variation due to temperature change is of the order of 4.0 nanoseconds (ns) per mile per .degree.C, while typical jitter variation is of the order of 9.0 ns per repeater stage. It is desired for all transmission loops terminating at the master exchange to send and receive all same-numbered channels within a frame period at substantially the same time. However, the time position (phase) of the incoming pulse data trains are subjected to fluctuations due to variations in the propagational conditions arising from cable temperature changes, lengths of particular transmission loops and a desynchronization effect of phase jitter. The temperature variable is a transmission phenomenon wherein the velocity of propagation along typical cables is a function of the cable temperature along the loop length. Phase jitter resulting from line repeater stations is a quick fluctuation as compared to the relatively slow temperature related change and has various causes such as crosstalk between transmission lines, the induction effect of parasitic periodical signals and the interactions between successively transmitted signals. The amplitude of the phase jitter is cumulative with repeater stations and hence increases in relation to the length of the transmission line. Where the initial difference or off-set between the WRITE/READ vectors is small or very close to zero, even small fluctuations in these propagational variables will cause overlapping.